Rotary dryer

ABSTRACT

A rotary dryer, which comprises an outer rotatable cylinder, a plurality of inwardly projecting outer paddles mounted to the inner surface of the outer cylinder, an inner rotatable cylinder of smaller diameter than the outer cylinder mounted coaxially within the outer cylinder, providing an annular space between the two cylinders for receiving material to be dried, a plurality of outwardly projecting inner paddles mounted to the outer surface of the inner cylinder, the inner paddles being spaced from the outer paddles, means for heating the inner and outer paddles, and means for rotating the inner and outer cylinders.

0 United States Patent 11 1 [111 3,824,704

Neville July 23, 1974 ROTARY DRYER 2,354,567 7 1944 Adt 34/134 1 Inventor Richard E. Nevins Salisbury, 5335333 121325 2 ;??5?::::...,......:1;1::::: 3321533 England [73] Assignee: AMF Incorporated, White Plains, Primary ExaminerCarroll B. Dority, Jr.

Assistant Examiner-Larry l. Schwartz [22] Filed: Aug 24, 1972 agorrgey, Agent, or Firm-George W. Price; Charles J. [2l] Appl. No.: 283,370

30 F A l P D [57] ABSTRACT 1 Drug pplca Tlonty am A rotary dryer, which comprises an outer rotatable Aug. 26, Great Bl'ltall'l Cy inder, a of inward y projec ing ou pad dles mounted to the inner surface of the outer cyling 34/134 165/ der, an inner rotatable cylinder of smaller diameter d 4 138 than the outer cylinder mounted coaxially within the 1 0 6 outer cylinder, providing an annular space between the two cylinders for receiving material to be dried, a plurality of outwardly projecting inner paddles [56] References Clted mounted to the outer surface of the inner' cylinder, the UNITED STATES PATENTS inner paddles being spaced from the outer paddles, 1,469,822 10/1923 Sauzedde 301/105 R means for heating the inner and outer paddles, and 1,486,405 35/1924 While 30l/105 B means for rotating the inner and outer cylinders. 1,796,651 3/l93l Harty et al.... 34/134 2,267,259 12/1941 Adt 34/134 8 Claims, 6 Drawing Figures PATENIED M23192:

SHEEI 1 [IF 5 1 ROTARY DRYER BACKGROUND This invention relates to drying apparatus and more particularly apparatus for drying particulate material in which the particles are small and have a high surface to volume ratio, such as cut leaf tobacco.

More specifically, this invention concerns improvements to rotary driers that include a cylinder lined with tubes or enclosed by a jacket and fitted with a number of internal radial lifting paddles extending for the length thereof. The cylinder and/or the paddles are heated by a heating medium such as steam or hot water passed through the jacket or tubes and material to be dried is progressed through the cylinder by inclining the axis of the cylinder slightly to the horizontal so that each time material is lifted by the paddle it is dropped slightly further down the cylinder. A rotary cylindrical drier of this, type is disclosed in British Pat. No. 1,209,929, published Oct. 21, 1970.

In dealing with particulate materials such as tobacco in a dryer of the above type, the drying time, which is of the order of minutes, is not determined by the rate of diffusion of moisture from the interior of the material to the surface as is the case with large particles having a low surface area to volume ratio, but by the rate of transfer of heat from the heated surfaces of the dryer to the material. If the heat transfer rate can be made high enough, the drying time can be lowered to seconds. I

The heat transfer rate and hence the rate of moisture removal is proportional to the surface area of the bulk material per unit weight in contact with the heated surface and the temperature difference between the material and the surface.

However, cut leaf tobacco and the like materials are of low bulk density due to the air spaces between the strands and are, therefore, good heat insulators. As a result, heat is transferred only to the surface layers of the material bulk so that the overall heat transfer and percentage moisture removal also depends on the number of times the material is tumbled in the dryer and fresh material surfaces are exposed to the heated surfaces.

The number of times the material is tumbled or dropped in the cylinder is dependent on the cylinder diameter, the length of the cylinder and the inclination of the cylinder to the horizontal. The time the material must be retained in the cylinder to dry is determined by the number of drops and the cylinder rotational speed. However, there is a centrifugal limit to the cylinder speed of a rotary dryer at which the material clings to the cylinder and no longer tumbles.

At the start of a drying process, the material thins out and spreads ahead of the main stream. Because the surface area per unit weight increases, the material overdries. Similarly, at the end of a run the material trails behind and thins out, again causing over-dried tobacco. The amount of over-dried tobacco is proportional to the contents of the dryer, which is in turn proportional to the dwell time, the time the tobacco remains in the dryer.

Accordingly, in order to keep the amount of material over-dried at the start and finish of the drying operation to a minimum, it is desirable to keep the dwell time to a minimum. A short dwell time is also desirable with respect to the automatic process control of the dryer and in order to cut down the amount of degradation or disintegration of the material particles due to tumbling.

Certain previous attempts to minimize dwell time have been unsatisfactory. For example, for a given size rotary cylindrical dryer which is running at the maximum speed, the dwell time can be reduced by increasing the inclination of the cylinder axis. However, this reduces the number of times the tobacco is tumbled and therefore it must be compensated for by an increase in surface area per unit weight if the same moisture removal rate is to be retained.

The surface area per unit weight can be increased by fitting more paddles to the interior of the cylinder and hence dividing the material into a greater number of streams. For example, doubling the number of paddles results in multiplying the surface area per unit weight by the square root of two. In practice, the dryer is designed with paddles at the maximum spacing to avoid the material jamming between paddles. Accordingly, doubling the number of paddles requires doubling the cylinder diameter. And, doubling the cylinder diameter is undesirable with fragile materials such as tobacco as it involves double the dropping height and therefore,

- greater degradation.

Alternatively, the cylinder diameter can be left unaltered and the surface area per unit weight increased by lengthening the cylinder while increasing the inclination of the cylinder axis to maintain a shortened dwell time.

However, greater cylinder lengths are undesirable because they are usually difficult to accomodate in a factory.

SUMMARY It is, accordingly, an object of this invention to provide an improved rotary dryer. I It is another object of this invention to provide a rotary dryer in which the moisture removal capacity is increased and/or the dwell time is reduced without increasing the heated length of the dryer.

It is still another object of this invention to provide a rotary dryer wherein the drying time and the tumbling height is reduced.

According to these and other objects, a preferred embodiment of the invention may comprise an outer rotatable cylinder, a plurality of inwardly projecting outer paddles mounted on the inner surface of the outer cylinder, an inner rotatable cylinder mounted coaxially within the outer cylinder, providing an annular space between the two cylinders for receiving material to be dried, a plurality of outwardly projecting inner paddles carried by the inner cylinder and spaced from the outer paddles, means for heating the inner and outer paddles, and means for rotating the inner and outer cylinders.

The material is dried in the annular space between the two cylinders. It is lifted by the paddles on the outer cylinder to around top dead center then tumbled onto the paddles of the inner cylinder which lowers it to around bottom dead center where it is tumbled back onto the outer cylinder.

By providing the inner cylinder with outwardly extending paddles, the tumbling height of the material is reduced, thus reducing degradation, and the heating area is increased since the number of paddles on which the tobacco is spread is increased by the additional paddles on the inner cylinder.

Since the tumbling height of the dryer is reduced, the annular cylinder may be inclined more steeply than a cylinder of the previous type of the same diameter. The increased angle of inclination and the increased heating area combine to reduce dwell time.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a view partly in elevation and partly in section of a dryer made in accordance with the invention.

DESCRIPTION OF PREFERRED EMBODIMENT A preferred embodiment of a rotary dryer according to the invention comprises an external stationary casing which is mounted on suitable supports disposing the casing at an angle to the horizontal. The angle of inclination may preferably be between 2 to 10 degrees. Tobacco is inserted into the cylindrical dryer at the higher end thereof, shown on the left in FIG. 1 and removed from the lower end, at the right of FIG. 1.

A pair of bearings, designated 12 and 13, are provided at the tobacco inlet and exit ends respectively and are mounted on support members 8 and 9 respectively. The bearings 12 and 13 mount hollow stub shafts l4 and 15 respectively for rotation. Bearing 12 is free to slide to allow for expansion while bearing 13 is fixed. An inner cylinder 19 is mounted on stub shafts l4 and 15 by spokes l6 and 17 respectively. The exit end of the cylinder 19 includes a drum 20 which is rotatable and is driven by a belt 21 (FIG. 2), worm box 22, and motor 23.

An outer cylinder 25 surrounds the inner cylinder 19 and is concentric therewith. The outer cylinder 25 is mounted with respect to the-inner cylinder 19 by a plurality of steel spokes 26 and 27.

The diameter of the inner cylinder is sufficiently smaller than that of the outer cylinder so as to leave an adequate annular space for the tobacco or other material to be heated therebetween. The inner cylinder, outer cylinder and annular space therebetween are disposed at an angle to the horizontal of between 2 and 10 Air seals 28, 29, 30, 31 and 32 are provided between the cylinders and adjacent stationary parts.

At the inlet end of the two cylinders is a cylindrical sieve 33 which forms a continuation of the outer cylinder 25. A vibrating tobacco feeding conveyor 34 introduces tobacco into the sieve 33.

The outer cylinder 25 is surrounded by heat insulating material 35 held in place by circular plates (not shown).

A plurality of semi-cylindrical pipe members 37 are welded to the inner surface of the outer cylinder 25 (see FIG. 4) and form channels therebetween.

A blade 38 is mounted to one side of every third pipe member 37 the blade extending radially inwardly toward the inner cylinder 19. A semi-cylindrical tube 40 is mounted on each of the blades 38. The pipe member 37, the blade 38 and tube 40 comprise the internal paddles on the outer cylinder.

A plurality of similar semi-cylindrical pipe members 41 are welded to the exterior surface of inner cylinder 19. A blade 42 is mounted on every second pipe member 41 and includes two semi-cylindrical tubes 43 mounted on one side face thereof. The pipe member 41, blade 42 and tube 43 form the external paddles on the inner cylinder. The pipes, tubes and blades of both cylinders are attached by welding.

Hot water is introduced to all the pipes and tubes by means of a rotary water inlet coupling 45 through stub shaft 14 to a distributor 46 and by a pair of circular pipes 47 and 48 to a pair of semi-circular distributor pipes 50 and 51. The water exhausts through a similar pair of semi-circular distribution pipes 52 and 53, a pair of circular pipes 54 and 55, a collector 56, an outlet pipe 57, a rotary coupling 58 and pipe members 59.

An aperture 60 is provided for the entry of hot air into the dryer. A perforated diffuser plate 61 is mounted over the aperture.

Within the sieve 33 are tobacco feed paddles 63 and surrounding the discharge end of the inner cylinder are discharge paddles 64 feeding to discharge aperture 65.

The spokes 26 at the feed end of the dryer are radially disposed between the inner and outer cylinders. One spoke is shown in FIG. 5 and is mounted at its ends in a manner which will allow for differential expansion of the two cylinders without imposing undue strain on the spokes of the two cylinders.

The outside and inside of the two cylinders carry respectively annular angle members and 71 welded to the cylinders. Two series of radially aligned holes 72 and 73 (only one hole of each series is shown) are provided through the members 70 and 71 and the cylinders, through which holes pass the spokes. The holes 72 and 73 are counter-sunk radially outwardly thereof. The spoke, which has threaded ends, is provided at its outer end with a frusto-spherical member 74 seating in the counter-sunk hole 72. Member 74 is held by a nut 75 and locknut 76.

On the inside of the'cylinder 25 the spoke carries a spring unit comprising a cup 77, dished spring washers 78 and a closure plate 79 all freely slidable on the spoke. The spring unit is held against the inner surface of the outer cylinder by a nut 80 secured by a lock nut 81.

At the inner end of the spoke a further frustospherical member 84 is provided.which seats in the counter-sunk hole 73. The member is held by a nut 85 and lock nut 86. A further spring unit is provided on the inside of the inner cylinder and again comprises a cup 88, dished spring washers 89 and a closure plate 90, all slidable on the spoke. A nut 91 engages the spring unit and is secured by a lock nut 92. Both spring units are adjusted so that the dished washers are under a set compression but capable of being compressed to a greater degree according to the differential expansion of the cylinders. If the outer cylinder expands, the inner spring unit, members88, 89 and 90, will be compressed to a greater extent while if the inner cylinder should expand, the outer unit, members 77, 78 and 79, will be further compressed. If both cylinders expand differentially, then the difference in spacing between them will be accommodated by the spring units while the spokes maintain adequate supporting connection between the two cylinders.

As seen in FIG. 2, the spokes 27 at the delivery end are mounted as if tangent to an imaginary circle having a diameter smaller than the diameter of the inner cylinder 19. Alternate spokes are disposed in a plane at right angles to the axis of the cylinders while the remaining spokes are inclined to said plane (see FIG. 1). The ends of the spokes are connected to the cylinders by ball connection type universal couplings, which are not shown in detail. The spokes accommodate expansion between the cylinders by permitting a small degree of torsional movement between the cylinders. The inclined spokes brace the outer cylinder against axial movement, any axial expansion or torsional movement being accommodated by flexing of the spokes at the feed end.

In operation, the tobacco is fed into the dryer by the conveyor 34 and enters the sieve 33, from where it is fed by paddles 63 to the space between the two cylinders 19 and 25.The tobacco is picked up by the heated internal paddles, comprising radial blades 38 and semicylindrical tubes 40, and carried up to near top dead center, whereupon the tobacco drops between the heated external paddles, comprising radial blades 42 and semi-cylindrical tubes 43 which carry it down to near bottom dead center where it drops again between the internal paddles. As the tobacco drops, it moves forward down the dryer until it is discharged by paddles 64 through apertures 65.

Hot air entering through aperture 60 passes through the space between the cylinders and passes out through the sieve 33, which prevents tobacco from being carried out with the air, and exhausts through an exhaust duct 68 (FIG. 3). Thus the air flow is counter to the direction of tobacco flow.

The sieve may be covered with gauze kept clean by an automatic steam cleaning device.

A small proportion of the air from the preheater is passed to the delivery end hood 20 to be mixed with the exhaust air. This avoids condensation in the exhaust air duct, particularly in high moisture removal applications, and permits the use of fabric exhaust air duct filters.

The casing carries suitable hot air ducting.

An alternative construction of the hot water distributor is shown in FIG. 6. The connections to the rotary unit 45 and the feed pipes 47 and 48 are as described with reference to FIG. 1. In this alternative construction, the two portions 46a and 46b of the distributor have been separated.

To the housing of the portion 46b a plate 94 is attached which carries a pair of rods 95, 96. Slidable on the rods are plates 97, 98 attached to the housing of the portion 46a, the latter being positional centrally of the plate 94 and a further plate 99 secured to the ends of the rods 95, 96. Bellows expansion joints 100, 101 are provided between plates 94, 97 and 98, 99 respectively. The bellows joint 100 serves to convey the heating water from the union 45 to the portion 46a of the distributor. By enabling the portion 46a to float on the rods, said portion can find a position where little or no stress is applied to the feed pipes 47 which would otherwise occur due to the differential expansion of the two cylinders.

1 An alternative form of drive to the two cylinders may be achieved by providing a pulley on the outside of the bearing (in a manner not shown). In such the hollow stub shaft 15 would be provided with a pulley between the bearing 13 and the rotary union 58.

Having now fully set forth both structure and operation of preferred embodiments of the concept underlying the present invention, it may be that various other embodiments as well as certain variations and modifications of the embodiments herein shown and described will occur to those skilled in the art upon becoming familar with said underlying concept. All such embodiments, variations, and modifications as incorpo rate the spirit of the invention and depend upon its underlying concept are consequently to be considered as within the scope of the claims appended herebelow, unless the claims by their language expressly state otherwise.

I claim:

I. A rotary dryer comprising an outer rotatable cylinder having a plurality of paddles extending inwardly from the inner surface thereof;

an inner rotatable cylinder having a plurality of paddles extending outwardly from the outer surface thereof;

said inner cylinder being of a smaller diameter than and mounted coaxially within said outer cylinder thereby providing an annular space between said cylinders for receiving material to be dried; the inner surface and paddles of said outer cylinder and the outer surface and paddles of said inner cylinder being provided with fluid conduit means for a heating medium;

means disposed in said inner cylinder and connected to said conduit means being adapted for connection to a source of a heating medium to supply such medium to said conduit means; and

means for rotating said inner and outer cylinders.

2. A rotary dryer in accordance with claim 1, and

said conduit means comprising a plurality of pipes mounted to the inner surface of said outer cylinder and the outer surface of said inner cylinder, and a plurality of tubes, and

each of said paddles comprising a blade mounted on one of said pipes and at least one of the tubes of said conduit means being mounted on said blade.

3. A rotary dryer in accordance with claim 2, and said means for supplying a heating medium to said conduit means comprising: 1

a distributor having a first portion, a second portion displaceable on rods carried at one end thereof by said first portion, and expansion joints between said first and second portions and between said second portion and the other end of the rods; and

feed pipes connecting said first portion to the conduit means provided to said inner cylinder and connecting said second portion to the conduit means provided to said outer cylinder.

4. A rotary dryer according to claim 1, wherein said annular space between the inner and outer cylinders includes an inlet and an outlet, a sieve is mounted in the inlet, and means are provided to feed material to be treated into the sieve means and means are provided to feed hot air to the outlet of the annular space.

5. A rotary dryer according to claim 1, wherein the inner and outer cylinders are mounted with their axes at an angle of between 2 to 10 to the horizontal, with the inlet end of the annular space therebetween higher than the outlet end.

6. A rotary dryer according to claim 1, wherein the inner and outer cylinders are fixed together for rotation in unison by means of spokes disposed between the two cylinders at the inlet and outlet ends thereof.

7. A rotary dryer as claimed in claim 6, wherein the spokes at the inlet end extend radially between the cylthe inner cylinder. 

1. A rotary dryer comprising an outer rotatable cylinder having a plurality of paddles extending inwardly from the inner surface thereof; an inner rotatable cylinder having a plurality of paddles extending outwardly from the outer surface thereof; said inner cylinder being of a smaller diameter than and mounted coaxially within said outer cylinder thereby providing an annular space between said cylinders for receiving material to be dried; the inner surface and paddles of said outer cylinder and the outer surface and paddles of said inner cylinder being provided with fluid conduit means for a heating medium; means disposed in said inner cylinder and connected to said conduit means being adapted for connection to a source of a heating medium to supply such medium to said conduit means; and means for rotating said inner and outer cylinders.
 2. A rotary dryer in accordance with claim 1, and said conduit means comprising a plurality of pipes mounted to the inner surface of said outer cylinder and the outer surface of said inner cylinder, and a plurality of tubes, and each of said paddles comprising a blade mounted on one of said pipes and at least one of the tubes of said conduit means being mounted on said blade.
 3. A rotary dryer in accordance with claim 2, and said means for supplying A heating medium to said conduit means comprising: a distributor having a first portion, a second portion displaceable on rods carried at one end thereof by said first portion, and expansion joints between said first and second portions and between said second portion and the other end of the rods; and feed pipes connecting said first portion to the conduit means provided to said inner cylinder and connecting said second portion to the conduit means provided to said outer cylinder.
 4. A rotary dryer according to claim 1, wherein said annular space between the inner and outer cylinders includes an inlet and an outlet, a sieve is mounted in the inlet, and means are provided to feed material to be treated into the sieve means and means are provided to feed hot air to the outlet of the annular space.
 5. A rotary dryer according to claim 1, wherein the inner and outer cylinders are mounted with their axes at an angle of between 2* to 10* to the horizontal, with the inlet end of the annular space therebetween higher than the outlet end.
 6. A rotary dryer according to claim 1, wherein the inner and outer cylinders are fixed together for rotation in unison by means of spokes disposed between the two cylinders at the inlet and outlet ends thereof.
 7. A rotary dryer as claimed in claim 6, wherein the spokes at the inlet end extend radially between the cylinders and are each connected at their inner and outer ends by spring units maintained in compression.
 8. A rotary dryer according to claim 7, wherein the spokes at the delivery end are arranged in the axial direction tangentially of an imaginary circle within the inner cylinder, alternate spokes being in a plane at right angles to the axis of the cylinders, the remaining spokes being inclined in the direction of feed to said plane to effect axial bracing of the outer cylinder in relation to the inner cylinder. 